Inductive current transformers, also known as current transformers (hereinafter also referred to as CT) for short, are widely used in electric measuring and relay protection. A CT transforms a higher primary current to a lower secondary current (or current in the secondary loop) with a certain ratio through core coupling based on the principle of electromagnetic induction, thereby reducing a high current of high-voltage power grid to a low current. A current transformer generally has closed core and windings, in which windings comprise primary winding and secondary winding. The primary winding together with the high-voltage loop are know as primary loop, while the external loop from the secondary winding of the CT to the measuring point is referred to as secondary current branch.
In order to transfer energy between the first and second windings, the CT has to first establish magnetic field. Such a magnetic field suffers from losses and a current known as exciting current is needed to maintain the magnetic field. The excitation characteristic of the secondary winding of the CT is a critical factor in evaluating the performance of the CT. Thus, the excitation characteristic experiment is one of the most effective and most widely used methods for testing the performance of protective current transformers. According to the excitation characteristic curve obtained from the excitation characteristic experiment, performance indexes for evaluating CTs as specified in IEC international standards “IEC60044-6 Instrument transformers—Part 6: Requirements for protective current transformers for transient performance” and “IEC60044-1 Instrument transformers—Part 1: Current transformers” can be measured. Those indexes may be, for example, composite error of the CT, accuracy limit factor (ALF) of the CT, secondary loop time constant Ts of the CT, knee-point, composite error, remanence factor, transient performance (for class-TP CTs) etc.
A conventional method for testing the excitation characteristic of CT is the power-frequency method, in which power-frequency voltage is applied across the secondary winding. The method works well for some CTs. However, class-P and class-TP protective CTs with a rated secondary current of 1 A are frequently used in practice, especially in extra-high and ultra-high voltage systems and large capacity generating sets. The knee-point voltage of such CTs is very high. When testing the excitation characteristic of such CTs, a power-frequency voltage of e.g. 2 kV or higher, even up to over 20 kV, is generally applied to the secondary winding. A large device capacity is required for applying such high voltage to the secondary winding. Meanwhile, the safety of operators of the experiments and the secondary wiring terminals can not be guaranteed by so high a voltage, which makes the experiment or test infeasible. As a result, the application scope of testing the excitation characteristic of the CTs using power-frequency method is limited.